EXPDP failed with ORA-39125: Worker unexpected fatal error in KUPW$WORKER.UNLOAD_METADATA

An expdp job failed with below error message:

ORA-39125: Worker unexpected fatal error in KUPW$WORKER.UNLOAD_METADATA while calling DBMS_METADATA.FETCH_XML_CLOB [OBJECT_GRANT:"WRHDBA01"]
ORA-04063: view "SYS.KU$_CLUSTER_VIEW" has errors
ORA-06512: at "SYS.DBMS_SYS_ERROR", line 105
ORA-06512: at "SYS.KUPW$WORKER", line 6313
----- PL/SQL Call Stack -----
  object      line  object
  handle    number  name
0x781f6ea8     15032  package body SYS.KUPW$WORKER
0x781f6ea8      6372  package body SYS.KUPW$WORKER
0x781f6ea8      2396  package body SYS.KUPW$WORKER
0x781f6ea8      6944  package body SYS.KUPW$WORKER
0x781f6ea8      1314  package body SYS.KUPW$WORKER
0x71f4f648         2  anonymous block
Job "SYSTEM"."SYS_EXPORT_SCHEMA_02" stopped due to fatal error at 17:17:49

In very many cases, an issue may raise multiple error messages, so it is very important to determine which message is the key and else are either useless or irrelevant.. In this case, obviously above red part error message is the key.

Let's check the view:

sbox >select owner,object_name,object_type,status from dba_objects where owner='SYS' and object_name='KU$_CLUSTER_VIEW';
OWNER      OBJECT_NAME                    OBJECT_TYPE         STATUS
---------- ------------------------------ ------------------- -------
SYS        KU$_CLUSTER_VIEW               VIEW                INVALID


It is INVALID, expdp query it and failed, hence we got the error. 
Let's check what make it Invalid:

sbox > select object_name,object_type,status from dba_objects where object_name in ( select REFERENCED_NAME from dba_dependencies where name='KU$_COLUMN_VIEW') and owner='SYS';
OBJECT_NAME                    OBJECT_TYPE         STATUS
------------------------------ ------------------- -------
COL$                           TABLE               VALID
OPQTYPE$                       TABLE               VALID
UTL_RAW                        PACKAGE             VALID
UTL_RAW                        PACKAGE BODY        VALID
DBMS_METADATA_UTIL             PACKAGE             VALID
KU$_LOB_VIEW                   VIEW                VALID
KU$_CONSTRAINT_COL_VIEW        VIEW                VALID
KU$_CONSTRAINT0_VIEW           VIEW                VALID
KU$_COLTYPE_VIEW               VIEW                VALID
KU$_OPQTYPE_VIEW               VIEW                INVALID
DBMS_METADATA_UTIL             PACKAGE BODY        VALID
KU$_SUBCOLTYPE_LIST_T          TYPE                VALID


We can see its base view KU$_OPQTYPE_VIEW is Invalid, and the base view is Invalid because:

sboc >select object_name,object_type,status from dba_objects where object_name in ( select REFERENCED_NAME from dba_dependencies where name='KU$_OPQTYPE_VIEW') and owner='SYS';
OBJECT_NAME                    OBJECT_TYPE         STATUS
------------------------------ ------------------- -------
OPQTYPE$                       TABLE               VALID
KU$_XMLSCHEMA_ELMT_VIEW        VIEW                INVALID

Next:

sbox >select object_name,object_type,status from dba_objects where object_name in ( select REFERENCED_NAME from dba_dependencies where name='KU$_XMLSCHEMA_ELMT_VIEW' and REFERENCED_NAME is not null);
no rows selected

We can see the base objects for  View 'KU$_XMLSCHEMA_ELMT_VIEW' not exist.
Below is the define for View 'KU$_XMLSCHEMA_ELMT_VIEW': We find out the root reason is some required XDB objects not exist. Install XDB may help to solve the issue.

sbox >select text from dba_views where view_name='KU$_XMLSCHEMA_ELMT_VIEW';
TEXT
--------------------------------------------------------------------------------
……
  from xdb.xdb$element xel, xdb.xdb$schema schm
……

sbox >DESC xdb.xdb$element
ERROR:
ORA-04043: object xdb.xdb$element does not exist

sbox >desc xdb.xdb$schema
ERROR:
ORA-04043: object xdb.xdb$schema does not exist

sbox >select owner from dba_objects where owner='XDB';
no rows selected

Install XDB should help in solving the issue.

Oracle XML Database (XDB) Install / Deinstall